Unittest added to Network Traffic Annotation Auditor.

Network traffic annotation auditor have no test support. A unittest is added to
cover some tests.

Bug: 656607
Bug: 690323
Change-Id: I93b05cf7ec4d89fca07166b230a4fc314fc438e7
Reviewed-on: https://chromium-review.googlesource.com/519244
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485225}

• BUILD.gn[diff]
• testing/buildbot/chromium.linux.json[diff]
• testing/buildbot/gn_isolate_map.pyl[diff]
• tools/traffic_annotation/auditor/BUILD.gn[diff]
• tools/traffic_annotation/auditor/traffic_annotation_auditor_unittest.cc[Added - diff]